1. Field of the Invention
This invention generally relates to an on-vehicle apparatus, and a method and a computer program for transmitting positional information. This invention particularly relates to a technology for transmitting information representative of the position of a vehicle to a positional information collecting apparatus.
2. Description of the Related Art
Japanese patent application publication number 2011-154494 discloses a traffic information collecting system in which on-vehicle apparatuses transmit probe information pieces concerning the travel trajectories or loci of related vehicles respectively, and a processing apparatus in a traffic information center collects the transmitted probe information pieces and detects traffic conditions in response to the collected probe information pieces. The processing apparatus also calculates travel times for respective road links from the collected probe information pieces.
A probe information piece generated by an on-vehicle apparatus has a sequence of samples each representing a point passed by the related vehicle and the time at which the point is passed by the vehicle. In the system of Japanese application 2011-154494, each on-vehicle apparatus selects, from samples, those representing points at or near ends of road links, and transmits only the selected samples to the traffic information center as a probe information piece. Thereby, it is possible to reduce the computation load on the processing apparatus which occurs during the calculation of travel times for road links.
A known on-vehicle apparatus repetitively records an information piece representative of the current position of the related vehicle at prescribed distance intervals or prescribed time intervals during the travel of the vehicle, and transmits the recorded positional information piece to a server. The known on-vehicle apparatus includes a memory for temporarily storing every positional information piece until it is transmitted. Generally, an on-vehicle apparatus can provide only a narrow space for such a memory, and the storage capacity of the memory is severely limited.
In the case where the vehicle having the known on-vehicle apparatus travels outside the service area about the server or in the event that the known on-vehicle apparatus falls incapable of communicating with the server, positional information pieces in the memory can not be transmitted while the amount of the positional information pieces in the memory gradually increases. When the memory is fully occupied by the positional information pieces, a new positional information piece can not be stored into the memory. It should be noted that the amount of the positional information pieces in the memory continues to increase until the on-vehicle apparatus resumes communications with the server.
It is conceivable to set relatively great the prescribed time intervals or the prescribed distance intervals for the repetitively recording of positional information pieces to delay a timing at which the memory becomes fully occupied by positional information pieces or to extend a distance to a point at which the memory becomes fully occupied by positional information pieces. In this conceivable case, it is possible to lower a probability that the memory will be fully occupied during a term from the moment of occurrence of a communication failure to the moment of occurrence of a communication recovery. On the other hand, as the prescribed time intervals or the prescribed distance intervals for the repetitively recording of positional information pieces are greater, a vehicle travel route calculated from the recorded positional information pieces is lower in accuracy.